Sponge-fauna of Norway. 439 



canal or oscular tube, transversely constricted by extensions 

 inwards of its walls, but not so completely as to acquire a 

 vesicular character ; the mouths of several tributary series of 

 vesicles immediately open into it ; and after proceeding for a 

 very short distance downwards, it completely disappears as a 

 tube, and is continued by several vesicular series, into which 

 it subdivides. 



Notwithstanding its wonderful transformation, the canal- 

 system is evidently homologous with that of the more normally 

 constituted sponges ; the sphincters which invariably occur at 

 the junction of any two vesicles are almost certainly an ex- 

 cessive over-development of the concentric rugaj which 

 characterize the canals of the Geodina and other sponges, and 

 which are more distinctly developed in the smaller branches 

 of the incurrent canals of Isops PJilegrcei (see antea^ vol. v. 

 fig. 1, p. 403). The first incurrent vesicle immediately 

 beneath the skin is situated in tissue characterized by the 

 absence of flagellated chambers, and is clearly homologous 

 with the ectochone of the Corticate. The second vesicle, so 

 far as its outer half, is similarly situated ; but its inner half is 

 brought into close communication with flagellated chambers ; 

 it thus represents an endochone and a subcortical crypt, while 

 the sphincter by which it opens into the first vesicle exactly 

 corresponds to that which we have termed the chonal sphincter. 

 It is easier to extract homologies from the modifications of 

 the canal-system than to find a use for them. Had the sponge 

 been a coast-dweller, subject to exposure between tides, one 

 might have regarded the vesicles and sphincters as a provision 

 for retaining a supply of water and thus guarding against 

 desiccation. But, so far from this, it is a characteristic deep- 

 sea form, exposed, as one would think, to but few changes of 

 condition. 



The vesicular enlargement of the canals certainly gives 

 them a larger capacity and superficial area, with a corre- 

 sponding diminution of the quantity of tissue in the sponge: 

 the volume of tissue is here at a minimum, of the canal- 

 system at a maximum ; thus the advantage is on the side of 

 increased food-supply, while the (Quantity of tissue to be fed is 

 diminished. Furthermore, not oidy does the vesicular ar- 

 rangement permit of a larger quantity of water being present 

 in the sponge at any given moment, but it facilitates a rapid 

 passage of water through it ; and, taking this fact along with 

 the abundance of large pores all over the sponge, it would 

 appear as though the most characteristic features of the canal- 

 system were in special adaptation to a free and rapid water- 

 streaming. We might then expect to find the body of the 



